The present invention relates to the treatment of heavy hydrocarbons, and more particularly to the refining of the heavy bottoms of petroleum.
Coking is a process in which the heavy residual bottoms of crude oil are thermally converted to lower-boiling petroleum products and by-product petroleum coke, and delayed coking involves the rapid heating of reduced crude in a furnace and then confinement in a coke drum under proper conditions of temperature and pressure until the unvaporized portion of the furnace effluent is converted to vapor and coke. Coke obtained by delayed coking from conventional residue feeds is almost pure carbon, called sponge coke, which is often employed in the production of electrodes for the aluminum industry, and special feeds produce premium coke, called needle coke, which is used in the manufacture of high quality graphitic electrodes important to the steel industry. A solvent deasphalting process is another of such heavy bottoms treatment processes, in which asphalt is removed from a feedstock, such as whole crude, atmospheric or vacuum residues, or any other heavy oil stream rich in asphaltenes, by the use of a solvent, such as propane, butane or other light hydrocarbons. In such a process, the feedstock is contacted with the solvent in an extractor, from which an asphalt mix containing asphalt and solvent are removed, and the asphalt is separated from the solvent in an asphalt recovery system. The extractor also produces a deasphalted oil mix of deasphalted oil and solvent, which is sent through a solvent recovery system including a deasphalted oil stripper before the deasphalted oil is sent on to refining as cracker feedstock (fluid catalytic cracker or hydrocracker).
Such processes yield by-products (coke or asphalt) from the heavy oil and valuable intermediate products for further refining in which primary products such as gasoline and gas oil are produced. With each process there are limits to the portion of the heavy bottoms which can be converted into the more valuable intermediate products, and so the remainder is converted to by-products. The processes require substantial amounts of energy to provide necessary heat, some of which is later lost, and they involve significant costs in equipment and piping. For example, the solvent deasphalting process involves the heating of the deasphalted oil mix, and requires the use of energy external to the process in order to supply the heat. In addition, the deasphalted oil taken from the solvent recovery system needs stripping before further use can be made of it, thus requiring equipment to perform the needed functions. In addition, some such processes produce waste material, which presents a pollution problem, since the waste material requires treatment and disposal. In the delayed coking process, the residence time in the delayed coker heater of the feed must be controlled to insure adequate heating of the feedstock while preventing the quick formation of coke deposits in the heater, since such deposits necessitate the shut down of the delayed coking apparatus while the heater is cleaned. Ordinarily such control of residence time requires the injection into the feed of a fluid such as steam or condensate to provide adequate flow velocities of the feedstock through the delayed coker heater. Such injection produces sour water and adds to waste treatment and disposal load associated with refining.